JPH07220301A - Optical pickup device - Google Patents

Abstract

PURPOSE:To always performs focus control in which offset is not included in a focus error signal and which has good precision. CONSTITUTION:Outgoing light A from a light source 1 is separated to luminous flux B for measuring forwarding to an object lens 4 side and luminous flux C for monitoring preventing to forward to the object lens 4 side by a luminous flux separating means 3, the luminous flux B for measuring is converged by an object lens 4 and an object to be measured 5 is irradiated with it, and its reflected luminous flux is led to a light receiving element 9 for measuring through an interference fringes generating means 8 having a first diffraction grating 8a and a second diffraction grating 8b. Also, by guiding the other side luminous flux C for monitoring to a light receiving element 11 for monitoring through an interference fringes generating means 10 for monitoring having a first diffraction grating 10a and a second diffraction grating 10b, a wave front of the luminous flux C for monitoring is compared with a wave front of the luminous flux B for measuring and a focus error signal including no offset is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device applied in the field of minute displacement measuring devices and the like.

[0002]

2. Description of the Related Art Conventionally, as an optical pickup device used for measuring a minute displacement of an object to be measured,
For example, there is a patent application filed by the applicant of the present application in Japanese Patent Application No. 5-330593 as "a minute displacement measuring device and an optical pickup device". FIG. 6 shows a configuration example of the device. Now, the light emitted from the semiconductor laser 1 as the light source is collimated by the collimator lens 2 and then reflected by the beam splitter 3 to obtain the objective lens 4
As a result, it becomes a focused beam and is irradiated onto the surface of the object to be measured 5. The reflected light from the object to be measured 5 again passes through the objective lens 4 and then passes through the beam splitter 3 to enter the double diffraction grating 6 as the interference fringe generating means. The double diffraction grating 6 differs from the first diffraction grating 6a that generates the first diffracted light of the ± n-order light and the second diffracted light of the ± m-order light that is different in grating pitch from the first diffraction grating 6a. It is composed of the generated second diffraction grating 6b. Then, by guiding the interference fringes generated between the second diffracted lights of the double diffraction grating 6 to the light receiving element 7, the phase and pitch of the interference fringes are measured. Accordingly, the displacement amount of the measurement object 5 in the X direction can be calculated by obtaining the periodic change in the light amount distribution on the surface of the light receiving element 7. For example, an in-focus state can be determined by using an optical disc as the measurement object 5 and obtaining the defocus amount of the disc surface from the focus error signal.

[0003]

The detection method of the above-mentioned device is basically a method of measuring the flatness of the wavefront of the interference fringes obtained by passing through the double diffraction grating 6. . Therefore, when the incident light on the objective lens 4 is a "perfect plane wave", the light spot is in focus on the disk surface when the value of the focus error signal is zero. However, in reality, it is impossible to completely make the incident light a plane wave, and in some cases, convergent light or divergent light may be intentionally made incident on the objective lens 4. In such a case, even if the value of the focus error signal is 0, it cannot be said that the light spot is in focus on the disk surface, and the focus error signal includes an offset, so that accurate signal detection can be performed. Can not.

[0004]

According to a first aspect of the present invention, a light beam is separated so that light emitted from a light source is separated into a measuring light beam that is directed toward the objective lens side and a monitoring light beam that is not directed toward the objective lens side. Means is provided, and the separated one of the measuring light beams is condensed by the objective lens to irradiate the object to be measured, and the reflected light beam is again in the optical path passing through the objective lens and the light beam separating means. The first diffracted light of ± n-order light is generated by the incidence of the light flux.
The diffraction grating and the first diffraction grating arranged to face the first diffraction grating.
And a second diffraction grating for generating a second diffracted light of ± m-order light when diffracted light is incident, and a measuring interference fringe generating means for generating an interference fringe by interference between the second diffracted light. A measuring light receiving element for detecting a change in phase and pitch of the interference fringes generated by the measuring interference fringe generating means is provided, and the other monitor separated by the light beam separating means before entering the objective lens. A first diffraction grating that generates a first diffracted light of ± n-order light when the light beam enters the optical path of the working light beam, and the first diffracted light that is arranged so as to face the first diffraction grating Second of ± mth order light
An interference fringe generating unit for monitoring, which has a second diffraction grating for generating diffracted light and generates interference fringes due to interference between the second diffracted light, is provided, and the interference fringe generated by the interference fringe generating unit for monitor. A monitor light-receiving element for detecting changes in the phase and pitch is provided.

According to a second aspect of the present invention, in the first aspect of the invention, the monitor light flux that does not go to the objective lens and the measurement light flux that is incident on the objective lens and reflected by the object to be measured are made the same by the light flux separating means. It was made to separate in the direction.

According to the invention of claim 3, claim 1 or 2
In the invention described above, the incident surface and the emission surface of the light beam separating means for separating the monitoring light beam that does not go to the objective lens and the measuring light beam that enters the objective lens and is reflected by the measurement object are arranged with an inclination with respect to the optical axis. did.

According to the invention of claim 4, claim 1 or 2
In the invention described above, the emitted light output control means for controlling the output of the emitted light from the light source is provided based on the output of the monitor light receiving element.

[0008]

According to the first aspect of the present invention, the emitted light, which is a light beam before entering the objective lens, is separated into a measuring light beam and a monitoring light beam by the light beam separating means, and one of the monitoring light beams is interfered with by the monitor. The offset was removed by comparing the wavefront obtained upon incidence on the fringe generation means with the wavefront obtained upon incidence on the measurement interference fringe generation means when the other measuring light beam was reflected by the object to be measured. It is possible to obtain the focus error signal.

According to the second aspect of the present invention, by separating the monitor light beam and the measurement light beam in the same direction,
1 for the monitor interference fringe generation means and 1 for the measurement interference fringe generation means
It is possible to configure with a single element, and accordingly, it is possible to configure both the monitor light receiving element and the measurement light receiving element on a single substrate.

According to the third aspect of the present invention, the incident surface and the emission surface of the light beam separating means are arranged so as to be inclined with respect to the optical axis, so that the internal reflected light of the light beam separating means and the monitor light beam or the measuring light beam are measured. It is possible to reduce the interference with the light flux.

According to the fourth aspect of the present invention, since the emitted light output control means is provided, it is possible to perform an external monitor for controlling the emitted light without newly providing an optical element for output control. It will be possible.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the invention described in claim 1 is shown in FIGS.
It will be described based on. The description of the same parts as those of the conventional example (see FIG. 6) is omitted, and the same parts are denoted by the same reference numerals. The overall configuration of this device will be described with reference to FIG. The emitted light A diverging from the semiconductor laser 1 as the light source is collimated by the collimator lens 2 and
The light enters the beam splitter 3 as a light beam separating means.
The emitted light A is split by the beam splitter 3 into a light beam B (measuring light beam) that is transmitted as it is and a reflected light beam C (monitoring light beam). One of the transmitted light flux B
Is condensed by the objective lens 4, is reflected on the surface of the optical disk 5 as the object to be measured, is incident on the objective lens 4 again, is reflected by the beam splitter 3, and is doubled as a measuring interference fringe generating means. It enters the folding grating 8. The double diffraction grating 8 includes a first diffraction grating 8a that generates a first diffracted light beam B1 of ± n-order light and a second diffraction grating 8b that generates a second diffracted light beam B2 of ± m-order light. ing. As a result, the light beam B reflected by the optical disk 5 is incident on the first diffraction grating 8a to generate the first diffracted light B1,
When the first diffracted light B1 is incident on the second diffraction grating 8b, the second diffracted light B2 is generated. Then, the interference fringes generated by the interference between the second diffracted lights B2 are received by the two-divided light receiving element 9 as the light receiving element for measurement, and the differential output F1 is obtained through the arithmetic circuit (not shown). Further, the other light flux C reflected by the beam splitter 3 before entering the objective lens 4 enters a double diffraction grating 10 as a monitor interference fringe generating means. This double diffraction grating 10
Is composed of a first diffraction grating 10a for generating a first diffracted light B1 of ± n-order light and a second diffraction grating 10b for generating a second diffracted light B2 of ± m-order light. As a result, the light beam C is incident on the first diffraction grating 10a to generate the first diffracted light C1, and the first diffracted light C1 is generated by the second diffraction grating 8a.
The second diffracted light C2 is generated by being incident on b. Then, the interference fringes generated by the interference between the second diffracted lights C2 are received by the two-divided light receiving element 11 as the light receiving element for monitoring, so that an arithmetic circuit (not shown) whose polarity is opposite to that of the arithmetic circuit is obtained. To obtain the differential output F0.

If the light spot focused by the objective lens 4 is in focus on the surface of the optical disk 5,
In principle, the wavefront of the light beam B reflected by the disk surface is the same as the wavefront of the light beam C before entering the objective lens 4. FIG. 2 shows a waveform of a curve of the differential output F1 which changes due to defocus and draws an S-shaped curve. At the S-shaped zero point (the value of the difference output F1 = 0), the wavefront of the reflected light beam B is completely flat, but this is not always the case when the spot on the disk surface is in focus. Therefore, assuming that the S-shaped value in the focused state is Q, the difference between P and Q is the offset Δ, and the value of Q is almost F0. Therefore, the difference output F0 and the difference output F1 should be compared. Thus, the focus error signal Fe without the offset Δ can be obtained.
As a specific example, assuming that the values of the sum signals of the two-divided light receiving elements 9 and 11 are S1 and S0, respectively, the focus error signal Fe
The value of can be calculated as Fe = (F1 / S1)-(F0 / S0) (1).

In this case, as shown in FIG. 3, it is assumed that a convex wavefront 12 is incident on the objective lens 4 on the lens side of the light beam B. At this time, the wavefront 13 incident on the monitor-side two-divided light receiving element 11 is in a convex state on the light-receiving surface side. The wave front 14 is concave on the light receiving surface side. Therefore, when the same arithmetic circuit is used for the two-divided light receiving surfaces having the same shape, the signs of the difference outputs F0 and F1 do not match because the detected wavefront unevenness has an inverse relationship. Therefore, the polarities of both arithmetic circuits are reversed. At this time, the wavefronts 13 and 14
Since both have a radius of curvature of several tens of meters or more and the distance L between the light receiving elements in the figure is about 100 mm (0.1 m), it is considered that the wavefronts are substantially parallel and have substantially the same radius of curvature. Therefore, the absolute values of the differential outputs F0 and F1 are substantially equal.

As described above, the outgoing light A, which is a light flux before entering the objective lens 4, is split by the beam splitter 3 into a measurement light flux B and a monitoring light flux C, and one of the monitor light fluxes C. Is incident on the double diffraction grating 10, and the wavefront 13 of the monitored light beam C is monitored.
And the wavefront 14 after the other measuring light beam B is reflected by the optical disk 5 and is incident on the double diffraction grating 8 by using the equation (1), the focus error with the offset Δ removed. The signal Fe can be obtained. Therefore, even if the light beam B entering the objective lens 4 is not a perfect plane wave, an offset does not occur in the focus error signal Fe and focus control can always be performed with high accuracy. . Further, even if the distance between the semiconductor laser 1 and the collimator lens 2 deviates over time, or the parallelism of the collimated light changes due to wavelength fluctuations or the like, the wavefront 13 on the emission side is constantly monitored to provide a focus error signal at the time of focusing. Since the value of Fe can be calculated, it is possible to cancel an offset that occurs due to a change with time or a temperature characteristic, so that focus control with higher reliability can be performed.

Next, an embodiment of the invention described in claim 2 will be described with reference to FIG. The description of the same parts as those in the first aspect of the present invention will be omitted, and the same reference numerals will be used for the same parts. In the present embodiment, claim 1
In the optical pickup device (see FIG. 1) of the described invention, as shown in FIG. 4, a monitoring light flux C that does not go toward the objective lens 4 and a measurement light beam that is incident on the objective lens 4 and reflected by the optical disk 5 The light beam B and the light beam B are separated in the same direction by a beam splitter 3 as a light beam separating means. Here, in order to separate the light beams B and C in the same direction, the beam splitter 3 is configured by using three triangular prisms.

By thus separating the monitor light beam C and the measurement light beam B in the same direction, the first and second diffraction gratings 8a and 8b of the double diffraction grating 8 and the double diffraction grating 8 are separated. 1
The first and second diffraction gratings 10a and 10b of 0 can be formed on one element (for example, one transparent substrate). Along with this, the two-divided light receiving element 11 for monitoring
It is possible to form both the light receiving surface of and the light receiving surface of the two-divided light receiving element 9 for measurement on one same substrate. Therefore, from the above, the number of diffraction gratings and light receiving elements can be reduced to about 1/2 of that of the embodiment of the invention described in claim 1, thereby significantly reducing the number of parts of the optical system. It is possible to provide a small, lightweight, and inexpensive device.

Next, an embodiment of the invention described in claim 3 will be described with reference to FIG. The description of the same parts as those of the first and second aspects of the present invention will be omitted, and the same reference numerals will be used for the same parts. In this embodiment, in the optical pickup device according to the first or second aspect of the present invention (see FIG. 1), as shown in FIG. 5, a monitoring light flux C that does not go to the objective lens 4 and is incident on the objective lens 4. Incident surface 3a of a beam splitter 3 as a light beam separating means for separating the light beam B for measurement reflected by the optical disk 5 into a light beam B for measurement.
And the emission surface 3b is arranged so as to be inclined with respect to the optical axis.

Specifically, in FIG. 5A, the incident surface 3a on the light source side of the cubic beam splitter 3 and the objective lens 4 are shown.
The light exit surface 3b on the side is arranged so as to be inclined with respect to the optical axis (note that in this figure, r _{1 to} r ₄ represent a part of the reflected light on each surface). In FIG. 5B, the emission surfaces 3c and 3 of the beam splitter 3 toward the two-divided light receiving elements 9 and 11 side.
d is inclined with respect to the optical axis. Figure 5
In (c), the incident surface 3e and the exit surface 3f of the light beam B of the beam splitter 3 toward the objective lens 4 are arranged so as to be inclined with respect to the optical axis. Therefore, by arranging the light-incident surface and the light-exiting surface of the beam splitter 3 with respect to the optical axis in this manner, the reflected light inside the beam splitter 3 and the light flux C for monitoring or the light flux B for measurement are separated. It is possible to reduce the interference between them, which makes it possible to obtain a more accurate focus error signal.

Next, an embodiment of the invention described in claim 4 will be described. The description of the same parts as those in the first to third aspects of the present invention will be omitted, and the same reference numerals will be used for the same parts. In this embodiment, in the optical pickup device according to the first or second aspect of the present invention (see FIG. 1), the output control of the emission light A from the semiconductor laser 1 is performed based on the output of the two-division light receiving element 11 for monitoring. Emission light output control means (not shown) for performing the above is provided.

Generally, as a method for controlling the LD (semiconductor laser 1), there is APC control for keeping the output power constant. In this APC control method, a PD (light receiving element) for monitoring the backward emission light of the LD is provided inside the element,
The APC operation at this time is performed based on the signal detected from the PD. However, such a method of monitoring the backward emission light has a drawback that it is easily affected by the return light.
Therefore, as in the present embodiment, the outgoing light A is controlled by the outgoing light output control means by using the light flux C which is the forward outgoing light, thereby performing accurate output control without being affected by the return light. You can Therefore, from the above, an external monitor for controlling the emitted light A can be performed without newly providing an optical element for controlling the output,
Moreover, by performing such external monitoring, the reliability for focus control can be improved.

[0022]

According to the first aspect of the present invention, there is provided a light beam separating means for separating the light beam emitted from the light source into a measuring light beam which is directed to the objective lens side and a monitor light beam which is not directed to the objective lens side. The separated one of the measuring light beams is condensed by the objective lens and applied to the object to be measured, and the reflected light beam again enters the optical path passing through the objective lens and the light beam separating means. ± n
A first diffraction grating that generates a first diffracted light of the next light, and a second diffraction grating that is disposed so as to face the first diffracted light and that receives the first diffracted light to generate a second diffracted light of ± m-order light. A measuring interference fringe generating means for generating an interference fringe due to interference between the second diffracted light having a grating, and detecting a change in phase and pitch of the interference fringe generated by the measuring interference fringe generating means. The first diffracted light of the ± n-order light is obtained by providing the measuring light receiving element, and causing the light beam to enter the optical path of the other monitor light beam separated before entering the objective lens by the light beam separating means. And a second diffraction grating that is disposed opposite to the first diffraction grating and that generates the second diffracted light of ± m-order light when the first diffracted light is incident on the first diffraction grating. Interference between two diffracted lights causes interference fringes Provided an interference fringe generating means for Nita,
Since the monitor light receiving element for detecting the change in the phase and pitch of the interference fringes generated by the monitor interference fringe generating means is provided, the emitted light is separated into the measuring light flux and the monitoring light flux before entering the objective lens. Then, the offset-removed focus error signal can be obtained by comparing the wavefront of the monitored light beam for monitoring with the wavefront of the measurement light beam reflected by the other object to be measured. Even if the wavefront of the measuring light beam incident on the lens is not a perfect plane wave, there is no offset and focus control can always be performed with high accuracy. In addition, even if the distance between the light source and the collimator lens deviates over time, or the parallelism of the collimated light changes due to wavelength fluctuations, etc., the wavefront on the outgoing light side is constantly monitored to determine the value of the focus error signal during focusing. Since the calculation can be performed, it is possible to cancel the offset caused by the change with time or the temperature characteristic at the same time, and thereby, the focus control with higher reliability can be performed.

According to a second aspect of the present invention, in the first aspect of the invention, a monitor luminous flux that does not go to the objective lens,
Since the measuring light beam incident on the objective lens and reflected by the object to be measured is separated by the light beam separating means in the same direction, the monitor interference fringe generating means and the measuring interference fringe generating means are formed by a single element. Since the monitor light receiving element and the measurement light receiving element can be formed on the same substrate, the number of components can be significantly reduced. A small, lightweight, and inexpensive device can be provided.

According to a third aspect of the present invention, in the first or second aspect of the present invention, a light beam separating beam which does not go to the objective lens and a measuring light beam which enters the objective lens and is reflected by the object to be measured is separated. Since the entrance surface and the exit surface of the means are arranged to be inclined with respect to the optical axis, it is possible to reduce interference that occurs between the reflected light generated inside the light beam splitting means and the monitor light beam or the measurement light beam, This makes it possible to obtain a more accurate focus error signal.
Further, this also reduces the return light to the light source.

According to a fourth aspect of the invention, in the invention according to the first or second aspect, based on the output of the monitor light receiving element,
Since the output light output control means for controlling the output of the output light from the light source is provided, an external monitor for controlling the output light can be performed without newly providing an optical element for output control. Thereby, the reliability of focus control can be further improved.

[Brief description of drawings]

FIG. 1 is a plan view showing an overall configuration of an optical pickup device according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing a state of a focus error signal that draws an S-shaped curve depending on a defocus state.

FIG. 3 is a plan view showing a state of a wavefront of each light beam separated by a beam splitter.

FIG. 4 is a plan view showing an overall configuration of an optical pickup device according to an embodiment of the invention as set forth in claim 2.

FIG. 5 is a plan view showing a state in which an entrance surface and an exit surface of a light beam splitting means that is an embodiment of the invention described in claim 3 are arranged so as to be inclined in each state with respect to the optical axis.

FIG. 6 is a plan view showing a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 3 luminous flux separation means 3a, 3e incident surface 3b, 3c, 3d, 3f exit surface 4 objective lens 5 measured object 8 measured object interference fringe generation means 8a first diffraction grating 8b second diffraction grating 9 measurement light receiving element 10 monitor Interference fringe generating means 10a First diffraction grating 10b Second diffraction grating 11 Monitor light-receiving element 15 Optical axis B1, C1 First diffracted light B2, C2 Second diffracted light

Claims (4)

[Claims] 1. A light beam splitting means for splitting a light beam emitted from a light source into a measuring light beam that is directed toward the objective lens side and a monitoring light beam that is not directed toward the objective lens side, and one of the separated measurement beams is provided. The object light beam is condensed by the objective lens and applied to the object to be measured, and the reflected light beam is again incident on the optical path that has passed through the objective lens and the light beam separating means. A first diffraction grating that generates one diffracted light, and a second diffraction light of ± mth order when the first diffracted light is incident on the first diffraction grating so as to face the first diffraction grating.
A second diffraction grating for generating diffracted light is provided, and measurement interference fringe generation means for generating interference fringes by interference between the second diffracted light is provided, and the interference fringe generated by the measurement interference fringe generation means. By providing a measuring light receiving element for detecting a change in the phase and pitch of the light beam, the light beam is incident on the optical path of the other monitor light beam separated before entering the objective lens by the light beam separating means. A first diffraction grating for generating a first diffracted light of the nth order light, and a second diffraction grating arranged to face the first diffracted light and to generate a second diffracted light of the ± mth order light by incidence of the first diffracted light. A monitor interference fringe generating means for generating interference fringes by interference between the second diffracted light having a diffraction grating is provided, and a change in phase and pitch of the interference fringes generated by the monitor interference fringe generating means is provided. Photodetector for monitor to detect An optical pickup device comprising: 2. A light beam for monitoring which does not go to the objective lens and a light beam for measurement which is incident on the objective lens and reflected by the object to be measured are separated by the light beam separating means in the same direction. 1. The optical pickup device described in 1. 3. An incident surface and an emission surface of a light beam separating means for separating a monitor light beam which does not go to the objective lens and a measurement light beam which is incident on the objective lens and reflected by the object to be measured, are arranged so as to be inclined with respect to the optical axis. The method according to claim 1 or 2, wherein
The optical pickup device described. 4. The optical pickup device according to claim 1, further comprising an emitted light output control means for controlling the output of the emitted light from the light source based on the output of the monitor light receiving element.